This invention relates generally to optical modulators, and more particularly to Stark effect modulators.
Modulation of a laser beam using the Stark effect generally utilizes a cell containing a confined gas having a molecular absorption resonance at or near the output frequency of the laser. The resonance can be frequency (or considered to be wavelength) tuned to become nearly coincident with the laser frequency by generation of a suitable electric field from application of a DC voltage across the Stark gas contained in the Stark cell.
Stark cell type modulation of optical (e.g. laser) beams generally utilize gaseous materials which have an absorption resonance at the laser frequency when the material is acted upon by an electric field. This effect occurs in both visible and infrared laser beams, but the process is most efficient with infrared lasers, such as CO2 lasers, or the like.
Most commonly applied to gases, the Stark effect involves the application of an electric field to a suitable material, the material having a substantial polarization. For example, varying the applied electric field results in changing the energy spacing of the molecular levels of the Stark material. The energy spacing changes the frequency or wavelength that is absorbed by the Stark material (e.g. Stark gas). The energy spacing of the Stark material is generally very small compared to the energy of the optical photons which interact with the Stark material. Application of an AC modulation signal in addition to an appropriate DC bias allows for modulation of the laser beam at the AC modulation frequency.
Stark effect modulation of laser energy is generally well known and described in numerous texts, articles and patents. For light having a wavelength of 9-11 xcexcm, which can be provided by a CO2 laser, an efficient molecule for Stark modulation is known to be ammonia (NH3 and its isotopes) due to its large Stark coefficient and strong absorption amplitude. This technique relies on a near coincidence in wavelength (or frequency) between at least one of the plurality of lines emitted by the CO2 laser and one of the absorption frequencies of the ammonia molecule. A modulation signal producing a time varying electric field is used to slightly change the ammonia absorption resonance into and out of coincidence with the laser beam frequency producing amplitude modulation by absorption.
Examples of Stark modulators for a CO2 lasers include U.S. Pat. No. 3,806,834 to Johnston et al. entitled xe2x80x9cStark Effect Modulation of CO2 Laser with NH2Dxe2x80x9d and U.S. Pat. No. 4,085,387 to Asawa et al. entitled xe2x80x9cStark-tuned Laser Modulatorxe2x80x9d. Asawa et al. discloses modulation of C13O2 laser light at 10:73 xcexcm and 10.78 xcexcm with NH3, while Johnston et al. discloses modulation of C12O2 laser light at approximately 10.6 xcexcm with NH2D, where D is deuterium, an isotope of hydrogen.
Stark cells described in both these patents and other available Stark modulators use free space propagation of the optical beam through the Stark modulator cell. Such modulators are constructed from two parallel electrodes separated by a distance, the separation distance being much smaller than the electrode width to minimize the variation of electrical field across the optical (e.g. laser) beam.
Conventional Stark cell designs use electrodes spaced 2to 3 mm apart, the electrodes being approximately 30 to 40 mm in length, or more. Since Stark cell length is approximately proportional to the degree of modulation provided, practical Stark cells generally require significant modulation lengths, often being hundreds of centimeters long. Electrode spacing is a parameter requiring a tradeoff for conventional Stark designs. Too small an electrode spacing results in excessive beam vignetting, while the necessary voltage to produce the required electric field intensity to align the desired Stark line with the incident laser beam increases as the electrode spacing increases.
Electrodes in conventional Stark cells are positioned within a vacuum enclosure, the vacuum enclosure filled with a Stark gas. As noted, these designs result in the loss of light within the modulator caused by vignetting of the optical beam by the exposed electrodes. In addition, such designs result in relatively slow modulation speed due to the high electrical capacitance and large voltages necessary for practical modulation efficiencies. High electrical capacitance results in part from the electrode plates being wider than the width of the incident laser beam to help provide a more uniform electric field throughout the modulation region.
A sketch of a conventional Stark cell modulator 100 is shown in FIG. 1. An optical beam 105, after being focused by lens 101, passes between parallel electrode plates 110 and 112. Vignetting of beam 105 by electrode plates 110 and 112 is shown by beam 105 overlapping electrodes 110 and 112 as the beam passes through the length of Stark cell 100. The output of Stark cell 100 is shown directed to a second lens 130. As shown in the inset, a vacuum enclosure 120 having suitable end windows (not shown) is provided to seal the Stark gas within the modulator 100.
Modulator 100 results in significant loss of light transmission through its length caused by vignetting of the optical beam (dotted line) by the exposed electrodes. It is difficult to focus long-wavelength infrared beams on the order of 10 xcexcm to form a small beam over a sufficiently long distance to avoid significant optical attenuation by the exposed electrode structure. Typically, these devices have been constructed for operation at low (audio) frequencies primarily for frequency stabilization of the laser in physics studies of the Stark effect itself. In addition to the optical loss caused by the electrodes, the long, wide, closely spaced electrodes have substantial electrical capacitance, preventing high frequency modulation and the ability to optically transmit data at high data rates.
An optical modulator includes a dielectric waveguide for receiving an optical beam and coupling energy of the optical beam into the waveguide, the optical beam being at a first frequency. At least one Stark material is disposed in the waveguide, the Stark material having at least one absorption frequency that is electrostatically tunable within a range of frequencies. The range of frequencies includes the first frequency. A bias circuit for generation of a bias electrical field across the Stark material is provided to shift at least one of the absorption frequencies towards the first frequency. A circuit is also provided for producing a time varying electrical field across the Stark material. The time varying field is adapted to shift at least one of the absorption frequencies towards the first frequency and away from the first frequency.
The bias field can have alternating field component and the alternating field component can be generated by a square wave signal. The alternating field component preferably has a frequency sufficient to switch polarity faster than the rate of charge build-up which can occur on the walls of the waveguide. The bias circuit can include two amplifiers, the bias amplifiers configured to produce outputs having opposite polarities in response to a given input signal. The circuit for producing a time varying signal can also include two high frequency amplifiers, the high frequency amplifiers configured to produce outputs having opposite polarities in response to a given input signal. A structure for synchronizing the polarity of the time varying electrical field with the alternating field component can also be provided.
The optical modulator can include conductive electrodes disposed on the outside of the waveguide. Thus, the waveguide isolates the optical beam from the electrodes. A CO2 laser can be provided for producing the optical beam, the Stark material preferably being ammonia. Ammonia can provide an enhanced concentration of deuterated ammonia (NH2D)
The waveguide can have a bore size of less than 1.0 mm. The modulator is efficient. Substantially all incident power of the optical beam can be coupled into an HE11 mode in the waveguide, such as at least 90%. The optical beam can be linearly polarized and oriented perpendicular with respect to the applied electrical fields. The waveguide can be quartz for a quartz waveguide, an alternating field component frequency is preferably at least 100 Hz.
The circuit for producing a time varying electrical field can generate at least one analog data component. The analog data component can be a chirped signal, the chirped signal varying over a frequency range. Analog data can be transmitted with the optical beam by variation of optical beam parameters such as amplitude, phase and frequency. The analog data component can include a plurality of sub-carriers, the plurality of sub-carriers multiplexed onto the optical beam, where each of the sub-carriers provide analog information.
The circuit for producing a time varying electrical field can generate at least one digital data component. The digital data component can include comprises a plurality of sub-carriers, the plurality of sub-carriers being multiplexed onto the optical beam, where each sub-carrier can provide digital information.
The modulator can be used in a variety of systems. For example, the invention can be used for a free-space optical link, laser radar, chemical detection, target illumination and active illumination for infrared imaging.
A method for modulating optical signals includes the steps of providing a dielectric waveguide for receiving an optical beam and coupling energy of the optical beam into the waveguide. The waveguide has at least one Stark material disposed therein, the Stark material having at least one absorption frequency that is electrostatically tunable within a range of frequencies. The optical beam is coupling into the waveguide, the optical beam being at a first frequency. The first frequency is within the range of frequencies. An electric field including a time varying field portion is applied across the Stark material. As a result, the optical beam output by the waveguide is modulated by the time varying portion of the electrical field.
The optical beam can be is provided by a CO2 laser and the Stark material can be ammonia. The ammonia preferably comprises ammonia having an enhanced concentration of deuterated ammonia (NH2D) The coupling step can be difficult. For example, substantially all incident power of the optical beam can be coupled into an EH11 mode in the waveguide, such as at least 90%.
The method can include the step of providing a bias field across said Stark material, where at least a portion of the bias field has an alternating field component. An alternating field component can be generated by a square wave signal. The alternative field component preferably has a frequency sufficient to switch polarity faster than the rate of charge build-up on the walls of the waveguide. Preferably, the waveguide is quartz and the frequency of the alternating field component is at least 100 Hz.
Substantially all of the bias field can be an alternating field. The bias field can be generated by a differential signal, the components of the differential signal having opposite polarities. Preferably, the polarity of the time varying electrical field is synchronized with the alternating field component. Time varying electrical field portion can be generated by at least one analog data signal. The analog data signal can be transmitted with the optical beam using variation of at least one optical beam parameter, such as amplitude, phase and/or frequency.
The analog data signal can include a plurality of sub-carriers. The sub-carriers can be multiplexed onto the optical beam, with each of the sub-carriers providing analog information. The time varying portion can be generated by at least one digital data signal. The digital data signal can include a plurality of sub-carriers. The sub-carriers can be multiplexed onto the optical beam, each of the sub-carriers providing digital information.